This invention relates to a polarization coupler for use for the introduction of light from two laser diodes into a single optical fiber or the like and also to a process of producing such polarization coupler.
In order to build up an optical fiber communication system of a high reliability, it is effective to duplex a light source of a laser diode or the like in an optical transmitter. In this instance, the two light sources are constructed in advance such that output beams of light therefrom may be introduced into an optical fiber, and at an initial stage after starting of operation of the system, only one of the two light sources is used, but after the light source fails, the other light source is alternatively used to prevent a possible system down. A polarization coupler is used to introduce, in a highly reliable system of the type mentioned, beams of polarized light from two light sources (linearly polarized light or elliptically polarized light approximate to linearly polarized light) into a common optical fiber. By the way, in an optical transmitter, part of an optical output is sometimes split and power of the thus split light is monitored in order to watch, for example, deterioration of a light source with the passage of time. A polarization coupler suitable for such monitoring is thus demanded.
Conventionally, a polarization coupler which includes a polarizing prism and a beam splitter having a branching ratio free from polarization dependency is known as a polarization coupler which allows such monitoring as described above. In an optical transmitter which includes a polarization coupler of the type just mentioned, beams of light emitted from, for example, two laser diodes and having polarization planes perpendicular to each other are collimated by a lens and introduced into a same light path by way of a polarizing prism so that they are introduced into an optical fiber by means of a beam splitter having a branching ratio free from polarization dependency and a condensing lens. Monitoring for the light intensity is effected for light split by the beam splitter.
Since a conventional polarization coupler is constituted using a polarizing prism for the composition of beams of light (linearly polarized light in ordinary cases) from two light sources while using a beam splitter having a branching ratio free from polarization dependency for the monitoring and besides requires a lens for forming a parallel light beam system in this manner, there is a limitation in miniaturization of the device.
Further, since very accurate adjustment in optical axis is required in assembly, the operability in production is not high.
In addition, since the branching ratio between main signal light and light for the monitoring is determined decisively by a branching ratio of the beam splitter, it cannot be varied readily.